As semiconductor chips increase in complexity, the need arises for an increased number of inputs and outputs. The result has been the development of new semiconductor packages such as the ball grid array (bga), the column grid array (cga) and similar structures which have large numbers of contacts.
There is a need for reliable, robust connections between each chip contact and the corresponding pad on the printed circuit board to which it is attached. Direct visual inspection of soldered contacts is impossible after assembly because the contacts are sandwiched between the board and the chip. Even if a defect is detected by such means as x-ray imaging, chip removal from a printed wire board is difficult and removed chips are not generally reusable.
Consider an individual chip with multiple leads which takes the form of an array of solder balls as shown in FIG. 1. Generally, if this chip is placed on a flat surface, such as a printed wiring board (PWB), only three balls will make contact with the board (three points define a plane). The particular three balls in contact depend on the chip mass distribution, the individual ball heights, and other parameters. Other balls may contact or nearly contact the PWB surface depending on chip tolerances in the size of the balls and flatness of the chip among other things. The closer to the surface all the balls are, the more reliable the interconnection of all the leads once the solder has been reflowed. The separation between the ball tips and the flat surface is referred to as coplanarity. The magnitude of the coplanarity parameter is a predictor of how well chip leads can be accurately and reliably soldered to their corresponding pads on a printed circuit board and as a diagnostic for determining when the elements of the package fabrication process are going out of control.
Coplanarity is generally measured optically since mechanical measurements can not be made with the necessary speed and accuracy. Three dimensional vision techniques, which yield values of surface height as a function of surface position, are preferred for inspection or statistical process control. There are a number of potential problems associated with optical measurement of coplanarity, however. When optical techniques are applied to the measurement of bga and cga substrates, erroneous results can result. The substrate material is generally a bismaleimide triazine (BT) epoxy laminate which is partially transparent or translucent at wavelengths used by three dimensional vision equipment and reveals embedded conductive lines and surfaces within the layer. These embedded elements can be mistakenly interpreted by the three dimensional vision system as defining the surface height. Since the geometry of the embedded elements is often proprietary to the semiconductor manufacturers who created it, the vision system can not even be pretaught to disregard particular data points.